This invention relates generally to composite porous membranes, and more particularly to composite porous membranes having oleophobic properties.
It is known that a porous membrane may have at least one property that is limited by the material that the membrane is made from. For example, a porous membrane made from an expanded polytetrafluoroethylene (ePTFE) material that is intended for use in garments and apparel has excellent hydrophobicity so it is considered to be waterproof at a relatively low challenge pressure. However, the ePTFE membrane tends to absorb oil. Such a tendency to absorb oil could affect the hydrophobicity in the area of the membrane that has absorbed the oil so that area of the membrane may no longer be considered waterproof.
U.S. Pat. No. 4,194,041 discloses a way to protect an ePTFE membrane from contamination by oil. A continuous hydrophilic film is attached to the ePTFE membrane to protect one side of the ePTFE membrane from oil. This structure is not air permeable and the hydrophilic film must contain moisture to transmit the moisture through the membrane. A heavier garment results from the necessary moisture present in the hydrophilic film. A person wearing a garment incorporating the membrane with the hydrophilic film often can feel uncomfortable because the hydrophilic film that contains moisture contacts the wearer's body, especially in cool environments. Such discomfort has been described as a “wet and clammy” feeling. This discomfort may be further aggravated by a lack of air moving through the garment that could serve to carry the moisture away from inside the garment.
U.S. Pat. No. 5,539,072 discloses the use of relatively small fluorinated acrylate particles to form a protective coating on a membrane. U.S. Pat. No. 5,976,380 discloses using a solution to provide a hydrophilic coating on a porous membrane. U.S. Pat. No. 5,156,780 discloses the in-situ polymerization of a protective coating layer on membrane.
U.S. Pat. Nos. 6,228,447 and 6,410,084 disclose an improved membrane structure that is air permeable to overcome the discomfort drawback described above yet protect the ePTFE membrane from oil contamination. A fluorinated acrylate oleophobic treatment is applied from relatively large particles in an aqueous dispersion in a manner so pores in the ePTFE membrane are not completely blocked. Air flow is permitted through the ePTFE membrane while it is protected from oil contamination. The effectiveness of the treatment is dependent on the particle size of the treatment material relative to the effective pore size in the ePTFE membrane.